The present invention generally relates to a pressure switch, and particularly relates to a pressure switch in which a diaphragm is held between an insulating cap and a conductive body so that an atmosphere chamber is formed by the insulating cap and the diaphragm and a pressure chamber is formed by the conductive body and the diaphragm, whereby pressure is detected by contact/discontact of a movable contact provided on the diaphragm with/from a fixed contact provided on the insulating cap when pressure in the pressure chamber becomes a predetermined value.
As such a pressure switch, conventionally, there has been a normally-closed type negative pressure switch having such a configuration as shown in FIG. 12, in which a contact opens/closes when negative pressure of, for example, an engine reaches to a predetermined value to thereby generate a signal for controlling the engine or the like.
An illustrated pressure switch 1 is constituted by a metallic conductive body 11 having a screw portion 11a to be screwed to a predetermined position of an engine, and an insulating cap 12 to be fit into a concave portion 11b of the conductive body 11 and integrated with the conductive body 11 through inward bending and caulking of the opening circumferential surface portion of the conductive body 11. A concave portion 12a is formed in the portion of the insulating cap 12 which is fit into the conductive body 11 so as to co-operate with the concave portion of the conductive body 11.
A diaphragm 13 is held, at its circumferential edge portion 13a, between the conductive body 11 and the insulating cap 12 which are integrated in such a manner as described above so that a pressure chamber 14 is formed by the diaphragm 13 and the concave portion 11b of the conductive body 11 and an atmosphere chamber 15 is formed by the diaphragm 13 and the concave portion 12a of the insulating cap 12.
In the center of the diaphragm 13, a center disc 16 acting as a movable contact exposed to the atmosphere chamber 15 is caulked, through a spacer 17, on the pressure chamber 14 side. The diaphragm 13, to which the center disc 16 is fixed, is urged to the atmosphere chamber 15 by a coil spring 19 which is inserted into a screw hole 11c of the conductive body 11 formed coaxially with the screw portion 11a and which is compressed in a loaded state between the diaphragm 13 and double nuts 18 screwed into the screw hole 11c. The spring force of the coil spring 19 is adjusted by the degree to which the nuts 18 are screwed. In order to ensure the looseness of the double nuts 18, a caulking member 18a is secured in the screw hole 11c, and a communicating hole 18b for applying engine negative pressure of, for example, a turbo car or the like is formed in the center thereof.
In addition, a fixed pin 20 is positioned, through insert formation, in the concave portion 12a of the insulating cap 12, and an external output terminal 21 having an L-shaped section and a cap-like conductive board 22 acting as a multipoint fixed contact are caulked on the fixed pin 20 through a spacer 23. The external output terminal 21 is projected partly outside so as to constitute, together with a connector housing 12b formed integrally with the insulating cap 12, a female connector for the electrical connection with a not-shown engine control circuit. In a hole 12c for leading the external output terminal 21 to the outside, an air vent groove 12c is formed so as allow make the atmosphere chamber 15 to communicate with the atmosphere. In addition, between the external output terminal 21 and the conductive board 22, a reception plate 24 is sandwiched so as to prevent water from adhering to a contact directly even if the water comes into the inside of the switch.
In such a configuration, when a relatively small amount of negative pressure is asserted on the pressure chamber 14 which is not sufficient to move the diaphragm 13 against the spring force of the coil spring 19, a movable contact constituted by the center disc 16 is in contact with a fixed contact constituted by the conductive board 22 as illustrated so that a switch constituted by these contacts is in a ON state. An electric path through the external output terminal 21, the reception plate 24, the conductive board 22, the center disc 16, the spacer 17, the coil spring 19, the nut 18 and the conductive body 11 to a body earth is therefore formed. When the engine negative pressure becomes large enough to move the diaphragm 13 to the pressure chamber 14 side against the spring force of the coil spring 19, on the contrary, the movable contact constituted by the center disc 16 moves together with the diaphragm 13 away from the fixed contact constituted by the conductive board 22 so that the switch constituted by these contacts is turned off to open the above-mentioned electric path. Thus, it is possible to detect the fact that the negative pressure becomes larger than a predetermined value.
In the above-mentioned conventional pressure switch 1, however, there has been the following problem. If water adheres to the entrance of the air groove 12c, the water enters the atmosphere chamber 15 directly through the air groove 12c as a result of intake operation caused by the movement of the diaphragm 13 because the atmosphere chamber 15 is made to communicate with the atmosphere directly through the air groove 12c formed along the external output terminal 21 in the insulating cap 12. The water entering the atmosphere chamber 15 intrudes into a mutual contact portion between conductive members to cause corrosion on the contact portion to generate a contact fault, so that the pressure switch erroneously operates to perform erroneous engine control.